DEVICE FOR EXAMINING OPTICAL WAVEGUIDES

Abstract: A device for examining optical waveguides comprises a prism (2) to be positioned on the waveguide to be investigated (1), a point (7) at which incoming light enters the device, as well as control accessories to alter the angle of light directed to the prism (2) from the entering point (7) of incoming light. The control accessories include a mirror (3) whereby the projection of the reflecting surface of the mirror in the plane of altering the said angle is in the form of an ellipse. A rotatable mirror (4) is placed at the focal point (F1) of the said ellipse, the corner of the prism (2) being situated at the second focal point (F2) of the same ellipse.

Description: Device for examining optical waveguides The object of the invention is a device for examining optical waveguides, which comprises a prism to be positioned on an optical waveguide under investigati¬ on, a point at which incoming light enters the device, as well as control accessories to alter the angle of light directed to the prism from the entering point of incoming light. Prism coupling is one of the most common methods to test integrated optical circuits and structures, and it is used in connection with methods to determine the effective refractive indices and thicknesses of the waveguides, by means of the structure of the propaga¬ ting modes of light within the waveguide. This techni¬ que is described in the communication Tien, P. K. , Ulrich, R. , and Martin, R. J. : Modes of propagating light waves in thin deposited semiconductor films, Applied Physics Letters 14 (1969) 9, pp. 291-294. Prism coupling is also required in measuring distribu¬ tions of propagating modes and attenuation in said structures. The device by means of which the testing is to be performed comprises a source of light, such as a laser, a prism, to the unbeveled corner of which the incoming ray of light is directed in a desired angle, as well as the measuring devices such as a light detector and a CCD-camera situated perpendicularly to the waveguide. The light must be directed to the prism at a characteristic angle pertaining to one of the propagating modes in the waveguide. This requires of the device that the source of light must be turned with respect to the prism, e. g. by means of a go¬ niometer. Since the source of light often is relative¬ ly large, its precise positioning places great demands on the fine mechanics of the device and at the least makes the whole device large and inflexible to varied geometries. The purpose of this invention is to remove the afore- mentioned drawbacks and present a device, by means of which the positioning can be done simpler and where the interchange of light sources can be simply perfor¬ med. In order to realize this purpose, it is in gene¬ ral characteristic of the invention that the control accessories include a mirror, whereby the projection of the reflective surface of the mirror in the plane of altering the angle is in the form of an ellipse, a rotatable mirror being placed at the focal point of the said ellipse and the unbeveled corner of the prism being situated at the second focal point of the same ellipse. The point at which the light from the light source is directed to the waveguide under investigati¬ on by means of the aforementioned mirrors, can then be constant within the device, and the angle at which the light enters the prism can then be altered by turning the rotatable mirror in the focal point of the ellip¬ se, around an axis perpendicular to the plane in which the angle is varied. Since the point of the rotatable mirror at which light is projected is situated at the focal point of the ellipse, the ray of light impinging on the elliptic mirror will always be reflected pre¬ cisely to the other focal point of the same ellipse, where the coupling corner of the prism in the prism coupler is located. The three-dimensional form of the mirror will also have options described further in the text. Since the optics of the device are mostly reflecting, the device is also independent of the wavelength of light, and different light sources can then be used without adjustments. According to one beneficial embodiment the device can be connected with the light source by means of an optical fiber. The connecting point can then be made precise with respect to the mirror permanently, and no adjustment of the light source in the prism coupling device need to be performed prior to measurements. According to one further beneficial embodiment, the device will have manipulators, such as a micrometer screw, a linear DC motor, a stepping motor, or a piezoelectric manipulator, which will be arranged to move the waveguide and the prism placed upon it, perpendicularly with respect to the direction of the light beam entering the device, so that the angle of the prism will stay at the other focal point of the ellipse. The virtue of this arrangement is that a larger area of the waveguide can then be investigasted at one time. In addition, other controls can be added in the device, by which the waveguide can be placed in positions relevant to imaging the waveguides. The invention is described in detail by reference to the Figures included, where Fig. 1 presents the device pertaining to the inven¬ tion perpendicular to the plane of turning of the light beams. Fig. 2 presents one beneficial embodiment of the device. Fig. 3 presents one alternative to the embodiment of Fig. 1, Fig. 4 presents one alternative to the embodiment of Fig. 2, Fig. 5 presents the reflection of the light beams according to one alternative, Fig. 6 presents some positioning alternatives of the waveguide. The device presented in Fig. 1 comprises a cylindrical prism 2, which is placed on a waveguide to be examined 1, so that an air gap of desired width will remain between the lower face of the prism and the upper surface of the waveguide. The prism will be positioned on the prism with accessories known in the art. A light beam 6 from a light source will be directed in the direction of arrow A through a narrow opening, such as a pinhole or a slit 7 to the focusing optics 8 and further through a corresponding second opening 9 to a rotatable plane mirror 4. A second opening can be situated after the mirror as well, to enable align¬ ment of the device. The said mirror is located in the focal point Fl of an ellipse defined by an elliptic mirror 3 so that the rotating axis of the mirror is situated exactly in the said focal point. Rotating the plane mirror enables the light beam from the light source passing through the focusing optics, also impinging at the focal point Fl, to be turned in varying angles to different points on the surface of the elliptic mirror 3, whence the beam will also be reflected in differing angles to the other focal point of the ellipse F2, where the unbeveled corner of the cylindrical prism 2 is located. The face of the prism directed towards the incoming rays of light is shaped in the form of a sector of a cylinder having its center of curvature at the second focal point F2. This shape and location of the cylindrical prism enables the device beneficially to be independent of light wavelength in the alignment, adjustment and operation of the device. The cylindrical prism may be manufactu¬ red of one piece or by attaching a part on the straight side of an ordinary prism, the outer face of the part being in the form of a cylinder surface. The projection of the reflective surface of the ellip- tic mirror 3 in the plane where the angle of incoming light is varied must have the shape of an ellipse. According to one alternative embodiment the reflecting surface is straight in a plane perpendicular to the main axis of the ellipse and the mirror is then simple to manufacture. The beam of light can thus be spread to a predetermined area. Alternatively, the mirror can be an ellipsoid according to Fig. 5, the reflecting surface being in this case in the shape of an arc in the said plane and thus the beam of light is focused in one point. The surface of the ellipsoid mirror is then in the shape of a rotational body which is ac¬ hieved by letting the ellipse rotate around its main axis. Fig. 5 shows the reflection schematically pro¬ jected to a plane perpendicular to the optical axis. The light beam from the light source through the focusing optics will be reflected in a diverging beam (exaggerated in the Figure for clarity) from the rotating mirror 4 to the reflecting surface of mirror 3. Because the focal point is in the center of a circle, the arc of which determines the reflecting surface in the said projection plane, the beam will again be focused by means of the reflecting surface also in a plane perpendicular to the optical axis to the second focal point F2 located on the main axis. When using an ellipsoid mirror, prism 2 is in the shape of a spherical surface against the direction of incoming light beams, and the second focal point F2 will then be located at the center of curvature de- fined by the spherical surface. A prism like this can be manufactured of a single piece or by attaching a separate piece with a spherical outer surface to the straight side of an ordinary shaped prism. Thus all rays of light reflected from the mirror 3 will enter the prism 2 perpendicularly to its outer surface. The plane mirror 4 can be rotated about its rotation axis by means of e. g. a micrometer screw, a DC motor, a stepping motor, or a piezoelectric manipulator. In addition, the prism 2 and the waveguide under investi¬ gation 1 can be translated perpendicularly with res¬ pect to the plane of turning of the light beams, and consequently, the unbeveled corner of the prism 2 remains in the focal point F2 and a wider area of the waveguide can be investigated. The translation can be realized by means of e. g. a micrometer screw, a DC motor or a stepping motor, or a piezoelectric manipu- lator. Fig. 6 presents alternatives to change the position of the waveguide, in order to have an imaging camera located otherwise than just above the waveguide as in Figs. 1 and 2. The change in the position of the waveguide is necessary also when the camera is heavy (e. g. a liquid nitrogen cooled CCD camera) and the waveguide must be imaged in different positions. The device has accessories to move waveguide 1 about an optical axis (the direction of incoming light to focal point Fl) . The parts rotating together in this case are at least parts numbered 1-4. The waveguide 1 and optics parts 2-4 are then situated in a separable (e. g. from an optical table 11) part 12 , which can be turned e. g. by 90 degrees or to another angle with respect to the optical table 11, and will be attached to the table in a manner allowing mirror 4 to remain at the optical axis. The table and the part have then suitable attaching accessories by means of which parts 1-4 can be positioned correctly. In addition, parts 1- 4 can be turned about an axis perpendicular to the optical axis in like principle. Thus, the beam of incoming light shall be turned from the original direction with a separate mirror 13 to the extent required by the angle of turning. Fig. 2 presents a device in the fashion of Fig. 1 that differs from Fig. 1 in how the light is brought into the device. The light is brought in by means of an optical fiber 5. The device shall then have a connec¬ ting point to attach an optical fiber connector 10. From the fiber connector the light is directed to the focal point Fl of the ellipse through a hole 7 and the focusing optics 8. The connecting point to attach the fiber connector 8 can be made precise in position with respect to the focal point Fl, and thus the light source can be changed without the need of aligning the beam of light over again. Fig. 3 presents an arrangement in the manner of Fig. 1, where the focal points Fl and F2 are not located on the same straight line as the optical axis 7 - Fl, but in contrast the main axis of the ellipse is in an angle with respect to the former. The situation is depicted in Fig. 4 using the arrangement shown in Fig. 2. At the location of the focusing optics, it is possible to arrange further a polarizer and/or other filters to change some properties of the incoming light. The signal can thus be enhanced and measurement possibili¬ ties be made more versatile, because when using a polarizer, the so called TE and TM modes are different in the waveguide. The said accessories can be located in a suitable position between the point of incoming light and the plane mirror 4.

Claims: CLAIMS : 1. A device for examining optical waveguides that comprises a prism (2) to be positioned on the wave- guide to be investigated (1), a point (7) at which incoming light enters the device, as well as control accessories to alter the angle of light directed to the prism (2) from the entering point (7) of incoming light, characterized in that the control accessories include a mirror (3) whereby the projection of the reflecting surface of the mirror in the plane of altering the said angle is in the form of an ellipse, a rotatable mirror (4) being placed at the focal point (Fl) of the said ellipse and the corner of the prism (2) being situated at the second focal point (F2) of the same ellipse. 2. A device according to claim 1, characterized in that the reflecting surface is straight in a plane perpendicular to the main axis of the ellipse. 3. A device according to claim 2, characterized in that the surface of the mirror (3) is an ellipsoid, and the reflecting surface has the shape of an arc of a circle in a plane perpendicular to the main axis of the ellipse. 4. A device according to any of the claims 1-3, cha¬ racterized in that the entering point (7) of incoming light comprises a connecting point for an optical fiber (5) . 5. A device according to any of the claims 1-4, cha¬ racterized in that between the entering point (7) of incoming light and the rotatable mirror (4) there is an accessory changing the property of light, such as a polarizer and/or a filter. 6. A device according to any of the claims 1-5, cha¬ racterized in that the device comprises manipulators, such as a micrometer screw, an DC motor or a stepping motor or a piezoelectric manipulator, arranged to move the waveguide (1) and the prism located thereon per¬ pendicularly to the direction of the beam of incoming light in a manner that the corner of the prism (2) remains at the second focal point of the ellipse (F2) . 7. A device according to any of claims 1-6, characte¬ rized in that the device has turning accessories to turn the waveguide (1) together with the mirrors (3, 4) and the prism (2) about the optical axis (7 - Fl) . 8. A device according to any of claims 1-7, characte¬ rized in that the device has turning accessories to turn the waveguide (1) together with mirrors (3, 4) and the prism (2) about an axis perpendicular to the optical axis.

Inventors: KIMMEL JYRKI, KIMMEL, JYRKI
Applicants: KIMMEL JYRKI, KIMMEL, JYRKI
Application year: 1994
Main classification: G01M11
IPC codes: G01M11